1. Field of the Invention
The present invention relates to a capacitive probe for measuring the level of an electrically conductive liquid in a vessel, particularly suited for high pressure and high temperature applications, and a method of manufacturing such a probe.
2. Description of the Prior Art
Examples of well-known methods intended to measure the evolution of the level of a liquid in a vessel are optical methods according to which the progress of a float is monitored by automatic control through a transparent window provided vertically in the wall. This type of measurement is suited only for clean liquids which do not affect the transparency of the window.
There are also ultrasonic methods where the level of the liquid in a vessel is measured by measuring the time of propagation of pulses reflecting on the surface. This type of measurement works well even at relatively high pressures and temperatures as long as the interface between phases is clear, but it loses precision when the liquid is highly emulsified.
It is also well-known to detect the position of the interface between an electrically conductive liquid by means of a capacitive probe whose capacitance is affected by the level variation.
Such a probe comprises for example a central metallic rod sheathed with an insulating plastic material, applied for example by heat shrinkage, which dips into the conducting fluid. The variation of the inter-electrode capacitance between the sheathed rod and the conducting fluid, resulting from the level variation, is measured. A probe of this type is for example used in the device which is described in French Patent 2,772,477 of the Assignee.
There are also level-measuring capacitive probes made up of a first electrode consisting of the metallic wall of a vessel which is separated from the conducting liquid by an insulating layer made of, for example, Teflon™. It can be the wall of a tube made of an insulating plastic covering the inner metallic wall of the vessel or an insulating coating (enamel for example) evenly deposited on the inner face. The inter-electrode surface is here the surface of the inner wall of tube 1. A probe of this type is used for example in the device described in French Patent 2,782,804 of the Assignee.
The capacitive probes mentioned above however have some drawbacks. First of all, it is difficult to deposit a uniform and thin insulating layer on a metallic rod or wall. The probes generally work properly under normal temperature and pressure conditions, but they deteriorate very quickly in more difficult environments notably as a result of creep effects affecting the plastic sheaths. This degradation occurs even when the insulating sheaths or coatings are made of materials withstanding heat, such as enamels for example. Enamel contains bubbles and thermal stresses cause microcracks that are invaded by the conducting liquid. The dielectric constant of the sheath or of the coating can thus vary considerably according to whether the sheath is more or less saturated with the liquid, which of course distorts the measurement results.